They're very strong. Eki would probably be my choice for best UI, 2f 2p 1c tiles are great in late Ancient, and then once you get Chivalry in early Medieval (which is a really strong tech that you'll want to beeline because it works with Attila's UA anyways) you get 3f 3p 1c which is fantastic, and adjacency bonuses only improve things. Their UA is also really nice when it comes to combat, oftentimes in war you'll capture a unit which your opponent will then have to waste a bunch of turns killing, and if not, hey, free unit. Usually I just let them heal up and use them as cannon fodder anyways since they start with 0 promotions but on rare occasions you can steal a UU.
In comparison, Horse Archers are pretty weak since you'll want to give priority to Horsemen that can benefit from your UA and capture stuff. They're not the worst UU but definitely below average, just one generic promotion and a couple CS/RCS above Skirmishers, which aren't an amazing unit for their time anyways (compared to Horsemen, which come half an era earlier and are slightly cheaper, Skirmishers have 12/10 CS versus 15). And no strategic requirements, but Horses are the one strategic that I almost never have issues with and unlike Egypt's UU they come at a time when you'll probably have your Horses improved anyways.
Very good UA, great UI, mediocre UU, overall I'd say it comes out to one of the best civs but probably not overpowered.